I've done may citizenship exam and received the results on the same day, however I don't understand the 'decision made' status which people are talking about. Is it the same as getting the test result or that's a different thing?
So after 'In Progress' status I should see the 'Decision Made' status and then I should expect to receive the oath letter? Or 'Decision Made' and oath letter are usually at the same time?
No they are usually not at the same time. I had my test on the 27th of July, decision made on the 28th of July and the oath invitation will take between 3 and 5 months, at least this is what we were told during the test. Very likely it is less than that.
It could also mean that they have decided NOT to let you get your citizenship, but I assume they would have advised you during the interview to withdraw the application if they saw any major mistakes.